This invention relates, in general, to radar systems and, more specifically, to signal processing equipment for scheduling between track and search modes in radar systems.
The rapid beam agility which is available with agile-beam or phased array radar antennas with electronic beam steering permits both accurate tracking of and searching for radar targets almost simultaneously. According to conventional practices, such radar systems use a portion of the radar operating time to search the entire region capable of being illuminated by the radar for the presence of targets, such as aircraft. Another portion of the operating time is used to track targets which have already been detected in previous search intervals. Since agile-beam antennas can be steered very rapidly, the radar operating time portions allocated to searching and to tracking can be closely spaced in time, with the differences according to current technology on the order of milliseconds.
Because of the ability to change between search and track modes rapidly, the manner of making such changes or scheduling becomes important and has been the subject of several U.S. patents. In general, these patents use the speed or acceleration of the targets to make a determination as to which mode the radar will use at any particular instant. The higher the speed or acceleration, the more important it is to track the targets more frequently, thus sacrificing searching time.
U.S. Pat. No. 3,858,206, issued on Dec. 31, 1974, discloses a system for scheduling between search and track modes in a track-while-scan radar. According to this patent, the frequency at which a target is required to be updated is determined as a function of target acceleration, distance to the target, and the size of the azimuth window containing the target. The calculated update time is compared to fixed values of 2, 4, 6, 8 and 10 seconds and the closest match is selected as the fixed time after which it is desired to update a target track. The higher priority targets are updated more frequently, or with less time between updates. After determining the fixed update time interval, it stores this value in memory associated with that target for future reference.
Every time the radar is ready to send out the radar signals, it looks at the data stored in memory and goes through a process to determine if the current signal should be a track or a search signal. It first processes the data to find the target most needing to be updated by looking at the ratio between the time since last updated and the desired update time. If the ratio is less than or equal to one, the radar goes into the search mode. If the ratio is greater than one, the track ratio is compared with a search ratio and, if the search ratio is smaller, the radar goes into the track mode for the target corresponding to the largest track priority ratio. If the search ratio is equal to or larger than the track ratio, the radar goes into the search mode. The search ratio is calculated from information dependent upon the number of targets needing to be tracked, a nominal search rate, and the time of the last search.
The present invention, unlike the referenced patent, uses target acceleration to classify all of the targets into a relative table of update frequencies which specify, not exact amounts of time, but frequencies by which the targets are updated in each radar cycle. Once classified, the radar cycle adheres to the relative update frequencies without any calculation of how long it has been since the last update of a particular target. Search dwells are inserted into the radar cycle at various intervals depending upon the time needed in the tracking mode to satisfy the assigned update requirements of the relative table. The relative update frequency is unaffected by the number or duration of the radar looks in a track or search dwell.
Therefore, it is desired, and it is an object of this invention, to provide a system for scheduling between track and search dwells in a track-while-scan radar system wherein the scheduling process matches the beam steering capabilities of a modern, agile-beam radar against the signal integration and track update rate requirements of multiple targets maneuvering at varying rates.